1. Field of the Invention
This invention relates generally to electronic devices and more specifically to a method and device for helping to predict failure rates in semiconductor devices. It is believed that mobile ion contamination is the phenomena which is being detected, but regardless of the reasons for failure, the method, devices, and circuitry described below can be used to help predict rates of failure of an integrated circuit device.
2. Description of Related Art
Positive ion contamination in semiconductor circuits located in the SiO.sub.2 and at the Si-SiO.sub.2 interface can cause instabilities in the operation of the devices and produce spurious results. Most commonly, the mobile ions that cause these problems are Na.sup.+ or P.sup.+. Several known methods exist to help detect this contamination, such as Bias-Temperature Capacitance-Voltage (BTCV), Bias-Temperature Current-Voltage (BT-IV), Triangular Voltage Sweep (TVS) and Thermally Stimulated Ionic Conductivity (TSIC).
On-chip testing circuits have been proposed to detect ion contamination. In particular, U.S. Pat. No. 5,394,101 issued to Mitros describes a method for detecting mobile ions through the use of a transistor with a floating gate. The transistor is. first biased to cause charge accumulation on the floating gate. The current through the transistor that corresponds to the accumulated charge is then measured. After the movement of mobile ions to the floating gate has been accelerated through a bake of the device, the current through the transistor (which now corresponds to the remaining charge on the floating gate) is measured. The difference between the two current values indicates the charge accumulation on the gate neutralized by the mobile ions. This approach, however, has the disadvantage of requiring an extra floating gate which increases the cost of the fabrication process due to added processing steps.
U.S. Pat. No. 5,619,459 issued to Gilliam describes another on-chip test circuit for detecting ion contamination. The circuit includes an enabling circuit, a voltage pump, and a regulating circuit. The test circuit is able to operate in two modes: normal and test. When in normal mode, the voltage of the substrate is held at an operating level via a voltage pump which becomes active whenever the substrate voltage drops below the operating voltage level. In the test mode, the voltage pump drives the substrate voltage toward the operating voltage while the regulating circuit shunts enough current to ground to hold the substrate at a different voltage between ground and the operating voltage, thereby allowing testing of the chip for mobile ion contamination without having to ground the substrate. Unfortunately, it is this voltage pump circuitry which causes problems for low voltage circuits (i.e., below three volts). Because the voltage pump reverse-biases the substrate, it may blow the gate of the circuit due to the fairly low voltage tolerance of the oxide gate in MOS circuits.
The method, devices and circuitry of the present invention provide additional reliable means by which ion contamination may be detected with the following advantages.